[clang][dataflow] Enable merging distinct values in Environment::join

Make specializations of `DataflowAnalysis` extendable with domain-specific
logic for merging distinct values when joining environments. This could be
a strict lattice join or a more general widening operation.

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: xazax.hun

Differential Revision: https://reviews.llvm.org/D118038